The study investigated the Effects of Computer Assisted Instruction on Achievement Mathematics among Junior Secondary School Students in FCT Abuja. The study has four objectives, answered four research questions and tested four null hypotheses. Quasi-experimental design was used as research design. Two co-educational schools were selected and designated as Experimental with 18 males and 18 females and Control groups with 17 males and 18 females. Reliability coefficient of 0.98 was obtained for TMAAT. Computer Presentation was developed, validated and used to teach the experimental group and thereafter TMAAT was administered to the students. Statistical Package for Social Sciences (SPSS) Version 20 was used to conduct the statistical analysis using t-test. The study revealed that there was significant difference between the mean achievement scores of students taught Algebra using Computer Presentation and those taught using lecture methods but there was no significant difference in achievement with regard to gender.